(1) Field of the Invention
The present invention relates to an image display apparatus which uses a light source unit to display an input image on a display panel.
(2) Description of the Related Art
In the case where the display device such as a liquid crystal display panel uses a backlight as the light source (light source unit) instead of emitting light by itself, the power consumption of the backlight occupies a major part of that of the display device. Reduction of the backlight power consumption is effective for reducing the entire power consumption of the display device. For example, amount of light from the backlight is reduced when displaying a dark image on the screen so as to reduce the power consumption. When the light amount of the backlight is simply reduced to the Nth part, the brightness of the screen may be reduced to the Nth part as well. If the light amount of the backlight is reduced to the Nth part while increasing the transmittance N times by compensating the value of each liquid crystal pixel to be brighter, the desired brightness on the screen may be maintained.
As to how far the light amount of the backlight may be reduced, that is, the upper limit of the value of N depends on the maximum value of the transmittance of the liquid crystal pixel (maximum transmittance). The N value may be adjusted to maximize the transmittance of the brightest pixel in the display image for maximizing the N to the extent that the brightness of the display image is not saturated. The method for collectively controlling the backlight luminance value (1/N) of the whole screen is called global dimming. N value is susceptible to the luminescent spot at least at one position existing on the screen in the global dimming, and accordingly, the luminance of the entire backlight is increased. The method may fail to provide the effect for reducing the power consumption in case of the image with high contrasting distribution on the screen.
In order to solve the aforementioned problem, so called local dimming (or area control) has been proposed (see H. F. Chen “Backlight Local Dimming Algorithm for High Contrast LCD-TV”, Proc. of ASID (2006), pp. 168-171 as Non-Patent Document 1). In the method, the screen is divided into plural regions, and each light source corresponding to the divided region one-on-one is provided for independently controlling the light emitting intensity of each of the respective light sources. The method applies the global dimming for each of the divided regions, and the light emitting intensity corresponding to the light source is determined based on the pixel value inside the region. The aforementioned process is executed for all the regions in the screen to control the backlight luminance in the respective regions independently. At the same time, the pixel value of the input image is compensated in accordance with the light emitting intensity of each light source of the respective regions. The aforementioned control is capable of reducing the power consumption more efficiently even if the input image has high contrast on the screen.
The local dimming is executed for controlling the backlight luminance in each of the divided regions. Difference in the backlight luminance between the regions may make the boundary therebetween visible. Especially when displaying the video image, not only the difference in the backlight luminance among the regions but also change in the backlight luminance of the respective regions along the time axis may cause flickering of the image because the image on the display screen fails to be smoothly switched apparently. Inappropriate control along the time axis may visually deteriorate the image quality. For example, U.S. Pat. No. 7,330,172 as Patent Document 1 proposes the control for changing output property so as to rapidly increase the luminance in response to the change in the display screen of the display panel from dark to bright, and to gently increase the luminance in response to the change in the display screen from bright to dark.